Solid adsorbents such as zeolites can be used in heat-to-power (HTP) and heat-to-chilling (HTC) sorption processes. In a HTP or HTC process, active agents in the form of gas molecules, such as CO2, adsorb onto the solid adsorbent at temperatures close to ambient at an initial pressure in a closed vessel. By introducing heat to the system, desorption of gas molecules occurs, which increases the number of gas molecules in the vessel producing a high pressure gas. The high pressure gas can be converted to power using a turbine and/or chilling through an expansion valve. To maintain continuous operation, the adsorbent must be then cooled down to a temperature close to the initial value. To make the HTC and HTP economically viable, the heating and cooling processes must be carried out in a very short period of time.
Heating and cooling the adsorbent from outside the vessel, using for example shell and tube geometry, requires a long period of time due to the low effective thermal conductivity between the heating/cooling fluid and the adsorbent and the large heat capacity of the shell and tube vessel and its contents.
Accordingly, there remains a need to make HTC and HTP processes utilizing direct heating and cooling of the adsorbent that reduce the cycle time needed to achieve temperature swings (ΔT) that are required to drive sorption systems (e.g., adsorption systems), reduce the overall size of the system, and more efficiently makes use of available heat sources available to the sorption system. There is also a desire to provide adsorbents that are non-reactive, or substantially non-reactive, toward the heating or cooling fluid so that adsorption of the gas onto the adsorbent is not unduly inhibited and the adsorbent is not degraded or destroyed by direct contact with the heating or cooling fluid.
US Patent Publication No. 2007/0092735 to Bruner et al., entitled “Polymeric Organometallic Films” discloses a method of applying a thick and durable self-assembled films or layers onto substrates for application in titanium orthopedic implants, organic light emitting diodes, ceramics, semiconductors and polymers. The films disclosed by Bruner are not suitable for use on porous adsorbent materials.
There is a need for a suitable selective nano layer coating for use on porous adsorbent materials (e.g., zeolite pellets) for protecting the adsorbent materials from fluids (e.g. water) while permitting an active gas (e.g., CO2) to penetrate the nano layer coatings and adsorb onto the adsorbent.
“Surface Modification of 13X Zeolite Beads for CO2 Capture From Humid Flue-Gas Streams” by Gang Li et al., discloses a method to produce a modified zeolite 13X having a better tolerance against water vapor, which normally exists in flue-gas streams compared to unmodified 13 X pellets against water vapor. The Si:Al ratio in the pellets is increased in order to obtain partial hydrophobicity. Gang Li et al rely upon the use of a coating that contains Si. Gang Li et al disclose a method consisting of coating zeolite 13 X with organosilane and then burning the coated pellet at a temperature greater or equal to 300 C for 10 to 15 hours. The high temperature treatment burns off the organic part of the coating and leaves additional Si on the pellets, thereby increasing the Si:Al ratio. The methodology makes the pellet partially hydrophobic and reduces the water vapor adsorption by 33%. As illustrated in Examples 14 and 15 described below, the approach outlined by Gang Li et al is not effective in producing a solid adsorbent that is capable of adsorbing CO2 while having no interaction with the liquid heating/cooling fluid.
There is a need for a hydrophobic nano layer protected solid adsorbent capable of adsorbing CO2 gas while having no interaction with a liquid phase cooling/heating fluid (e.g., water or triethylene glycol (TEG)) whereby the highly hydrophobic protected layer keeps water or TEG away from the surface reducing the interaction between the pellets and the liquid to zero.